Properties of Ionic Compounds
Do you think table salt, iron rust, baking soda, and limestone are very much alike? If you
answer no, you're right. If you answer yes, you're right, too! You wouldn't want to season
your food with rust, or construct a building out of baking soda. But despite their
differences, these compounds share some similarities because they all contain ionic
bonds. The characteristic properties of ionic compounds include crystal shape, high
melting points, and electrical conductivity.
Crystal Shape The object in the photo below that looks like a glass
sculpture is really a chunk of halite, or table salt. Halite is an ionic compound. All halite
samples have sharp edges, corners, and flat surfaces. These properties result from how
the ions are arranged. In solid sodium chloride, the Na+ and Cl- ions come together in an
threedimensional arrangement called a crystal.
alternating pattern, as shown in the diagram. The ions form an orderly,
Halite Crystal Arrangement A halite crystal contains sodium and chloride ions in an alternating
pattern. Making Generalizations What general characteristics of crystals can you observe in the
photograph of halite?
In an ionic compound, every ion is attracted to ions near it that have an opposite
charge. Positive ions tend to be near negative ions and farther from other positive ions.
doesn't bond with just one
negative chloride ion. It bonds with ions
As a result, a positive sodium ion
above, below, and to all sides. Because
chloride ions bond with sodium ions in the same way, a crystal
forms. This pattern continues no matter what the size of the crystal. In a single grain of
salt, the crystal can extend for millions of ions in every direction. The number of sodium
ions and chloride ions in the crystal is equal. The formula for sodium chloride, NaCl,
represents this 1 : 1 ratio.
High Melting Points
What happens when you heat an ionic
compound such as table salt? Remember, the ions are held together in a crystal by
attractions between oppositely charged particles. When the particles have enough
energy to overcome the attractive forces between them, they break away from each
other. It takes a temperature of
801°C to reach this energy for
table salt. Ionic bonds are strong enough to cause all ionic compounds to be solids
at room temperature.
Electrical Conductivity
When ionic compounds dissolve in water, the solution conducts electricity. Electricity is
the flow of electric charge, and ions have electric charges. However, if you connect wires
from a salt crystal to a battery and a light bulb, don't expect anything to happen. A solid
ionic compound does not conduct electricity very well. The ions in the crystal are tightly
bound to each other. If the charged particles do not move, electricity does not flow. But
what if the ions are broken apart? When ionic compounds
dissolve in
water, the ions separate. These ions then move freely, and the solution conducts
electricity.
Melting ionic compounds also allows them to conduct electricity. Can you figure out
why? Think about the difference between the particles in a solid and a liquid. In a solid,
the particles do not move from place to place. But in a liquid, the particles slip and slide
past each other. As long as the ions can move around, electricity can flow.